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• A trapped diamagnetic particle in an inhomogeneous magnetic field is studied analytically. • Compact expressions for the frequency and eddy-current-related Q-factor are derived. • The Q-factor grows with reduction of the particle size and the magnetic susceptibility. • Usefulness of the pendulum for the gravity constant measurements is highlighted. We discuss eddy-current-induced limitations of the attenuation of mechanical motion of a diamagnetic particle trapped by an inhomogeneous magnetic field in the Earth's gravitational potential. We show that the mechanical frequency of the particle oscillation is independent on the particle properties and is proportional to the free fall acceleration constant, similarly to the classical mechanical pendulum. The frequency can be used to measure the gravity field. The eddy-current induced attenuation constant does not depend on the mass of the particle and reduces with the particle volume. The quality factor of the mechanical motion can be as high as 109 and is comparable with the attenuation due to interaction of the particle with incompletely evacuated air. A possibility of usage of the particle as a quantum mechanical system is discussed. [ABSTRACT FROM AUTHOR]